MOLECULAR ANALYSIS OF THE ERWINIA-CHRYSANTHEMI REGION CONTAINING THE KDGA AND ZWF GENES

The pathways of pectin and galacturonate catabolism in Erwinia chrysanthemi converge to form a common intermediate, 2-keto-3-deoxygluconate, which is phosphorylated to form 2-keto-3-deoxy-6-phosphogluconate (KDGP) and then cleaved by the aldolase encoded by the kdgA gene. We cloned the kdgA gene of the E. chrysanthemi strain 3937 by complementing an Escherichia coli kdgA mutation, using an RP4-derivative plasmid. Restriction mapping of the kdgA region and isolation of kdgA-lac fusions allowed the more precise localization of the kdgA gene and determination of its transcriptional direction. The nucleotide sequence of the kdgA region indicated that the kdgA reading frame is 639 bases long, corresponding to a protein of 213 amino acids with a molecular mass of 22187 Da. Comparison of the deduced primary amino acid sequences of the E. chrysanthemi KDGP-aldolase to the E. coli, Zymomonas mobilis and Pseudomonas putida enzymes showed that they are highly conserved. The E. chrysanthemi kdgA structural gene begins 153 bases downstream of an open reading frame that has a high homology with the zwf E. coli gene encoding glucose-6-phosphate dehydrogenase. The zwf gene is also linked to eda (kdgA) in E. coli and P. putida but genetic organization is different. Regulation of zwf and kdgA expression in E. chrysanthemi was analysed using lacZ fusions. The expression of zwf is independent of the growth rate, but is repressed in the presence of glucose. Induction of kdgA by pectin-degradation products is mediated in vivo by the negative regulatory gene kdgR, which also controls all the steps of pectin degradation. However, the KdgR repressor is unable to bind in vitro to the 5'-untranslated end of the kdgA gene.